The 7-6.5 PRC isn't magic. I switched to it, from a .284-based chambering, for two reasons:
1. Better brass, and,
2. Greater flexibility
Lapua 6.5 PRC brass is excellent. Really, really good stuff. As good as the .284 Win and variations on that case perform, that rebated case is a bit of a weak spot. The Lapua magnum case is hell-for-strong, very consistent and is advantaged with lower bolt thrust. ADG seems to be every bit as good as Lapua 6.5 PRC brass and is a load-and-shoot proposition. Advantage 7-6.5 PRC.
With respect to flexibility, one can choose to run the PRC-based case at .284 Win. velocities, get excellent results on target and be secure in the knowledge the brass will last a very, very long time. Or, if the wind is expected to blow, the 7-6.5 can easily reach velocities that will trash .284 Win. brass in short order. Splitting the difference, the PRC case will easily handle the 183/185/190/195 gr bullets more capably and comfortably than the .284 Win. case. 190s at 2,900 fps is not a problem for the 7-6.5. In a .284, you'll be prepping new brass in short order.
Either way, you need to make good wind calls. If your waterline 9s are equally divided between upwind and downwind with a .284, your scores will not be materially different with a 7-6.5 at a higher velocity. Unless you shoot a bunch of 9s just a hair's breadth outside the lines with a .284, your scores won't change much.
To sum it up, the marginal cost of going with the magnum case is a trifle, ignoring all other considerations. The additional flexibility makes it a wash at the very worst. That's why I went with the 7-6.5.
